This invention generally relates to a method and device for determining the dielectric constant of a fluid using the time it takes a signal to travel along a probe that is inserted at least partially into the fluid.
There are a variety of situations where determining the dielectric constant of a fluid is necessary or would be useful. For example, diesel engines can develop cracks or leaks in cylinder liners, head gaskets and other components because of the heavy-duty work performed using diesel powered vehicles. Any one of these problems may present the potential for water to contaminate the oil in the engine. It is important to quickly detect the presence of water within oil because that presents potential engine failure or damage problems. Therefore, it would be useful to determine the dielectric constant of the oil on a routine basis so that the presence of water could be easily determined. Other uses for determining dielectric constants include determining mixtures of fuels and other liquids where the exact content of the liquid may vary. Another example would be to determine the quality of engine oil to facilitate changing the oil at an optimal time.
A number of proposals have been made for determining the dielectric constant of a fluid. For example, U.S. Pat. No. 3,812,422 describes a method that measures a dielectric constant based upon an amplitude of a reflected signal that reflects from an interface between different media such as air and fluid. The amplitude of the reflected signal provides impedance information that then is utilized to calculate the dielectric constant.
Although such a solution may be workable, it is not without shortcomings and drawbacks. A system that utilizes the amplitude of such a reflected signal requires using a variety of components including a peak detector with reset circuitry, a precision reference, and an analog-to-digital converter. Obviously, incorporating these various components can become cumbersome and overly expensive for some applications. Moreover, a system that relies on the amplitude of such a reflected signal is not as accurate as required for many applications because the measurements and the associated components needed to take those measurements are sensitive to temperature and other drift phenomena. In the diesel engine example described above temperatures can become very high. A further problem becomes evident in trying to compensate for signal drift in such systems.
This invention addresses the need for providing a method and device suitable for measuring a dielectric constant of a fluid in a variety of applications. This invention is especially useful and advantageous in situations where the level of a liquid also is determined.